1. Field of the Invention
The present invention is related to a novel method of removing scale in a cooling water system. More particularly, the present invention is related to a method of efficiently removing scale by using a scale removing agent for a cooling water system comprising isoprene sulfonic acid copolymer which has excellent ability to remove scale and does not easily form gel. The method also allows removal of scale without stopping operation of cooling water system while the ordinary operation of the cooling water system is maintained.
2. Description of the Prior Art
On heat transfer surfaces of boilers or cooling water systems, various kinds of hazard on the operation, such as decrease of heat efficiency, local corrosion and the like, occur often because of formation of scale, accumulation of sludge and the like other causes. It is known that the scale and the sludge are formed by concentration of impurities in water, such as calcium, magnesium, iron, silica and the like, which under conditions of high temperature lead to a decrease of solubilities of calcium carbonate, hydroxy apatite, iron oxides, magnesium silicate and the like and and precipitation of these compounds on the heat transfer surfaces of water side.
Furthermore, refrigerators have recently advanced recently in their packaging and higher performance and, because of this advancement, the rate of vaporization per unit heat transfer surface is highly enhanced in comparison with that of refrigerators of former types. By the increase of heat load on the heat transfer surface as described above, impurities in water are concentrated to a higher degree. Thus, scale is easier to be formed than before and slight formation of scale leads to increase of consumed electric power or steam and decrease of cooling efficiency, making the scale hazard a more important problem.
As the method of removing scale such as described above, a method of cleaning with an inorganic acid, such as hydrochloric acid, and a method of using a water soluble carboxylic acid copolymer are known. The method of cleaning with an acid has drawbacks that the operation of the cooling water system must be stopped for a while because a cleaning apparatus is added to the system and the cleaning operation is conducted while operation of the system is stopped, that the cleaning operation takes a long time and that there are other problems like corrosion of water systems and danger during handling.
On the other hand, in the method of using a water soluble carboxylic acid copolymer, although stopping of the operation of cooling water system is not required and the problem of corrosion of instruments and danger during handling are reduced, the content of monomer units having a carboxyl group, such as acrylic acid, maleic acid and the like, in the copolymer has to be increased for enhancing the abilities to dissolve scale like calcium carbonate and to prevent precipitation of scale. However, there arises a problem that a higher content of the monomer units having a carboxyl group in the copolymer results in higher tendency of the copolymer to form gel.
As the scale removing agent comprising a carboxylic acid copolymer, a scale removing agent comprising a copolymer of itaconic acid and other unsaturated carboxylic acid or unsaturated monomers other than the unsaturated carboxylic acid was proposed (Japanese Patent Publication 1986-47195). This agent has drawbacks that it forms gel easily and that operation cost is high in comparison with the ability to remove scale.
It is described in Japanese Patent Application Laid Open No. 1990-9496 that a polymer of sulfonated product of a conjugated diene is used as a cleaning agent for pipes.
A method of removing scale without stopping operation of cooling water system while the ordinary operation is maintained (On Stream Scale Removal) has been desired. As the method of cleaning verdigris without stopping operation of cooling water system, a method of using verdigris removing agent comprising hydrazine has heretofore been known (Japanese Patent Publication 1988-45474). However, scale of calcium compounds cannot be removed by this method. Because the inside of a heat exchanger cannot be observed directly in the method of removing scale in a cooling water system, general practice is that a cleaning agent is added after the operation of the cooling water system is stopped and, after the cleaning for a specified time, the cleaning water is brought into a laboratory to measure concentration of calcium by using atomic absorption spectrum method or the like. However, the evaluation of the effect of the cleaning by this method takes a lot of time and occasionally effects adversely on stable operation of a plant or an air conditioner. Thus, development of a method of evaluating the effect of cleaning on the spot of the operation in a short time has been desired.